A fixed expense is one for which the amount is same, and it should be considered planning other expenses.
A variable expense is one for which the amount is changing and it should be considered saving fixed expenses.
Discretionary spending is the most irregular budget item and the easiest to change.
Explanation:
Fixed expense - Amount is fixed
Variable expense - Amount is changing as per needs
Discretionary expense - Non-essential budget amount as per wants
Budget - A plan to save and spend expenses or cash-flow and managing them by writing them down regularly
